IMS OSAM Help

An OSAM structure may optionally be used for caching data. If data caching is not used, the structure contains only directory entries. If data caching is used, the structure contains directory entries and data elements. In this case the size of the structure must allow for data elements. OSAM data is stored in the structure as multiples of 2 KB data elements.

IMS/ESA OSAM uses directory-only cache structures to maintain local buffer consistency. Buffer pool coherency guarantees that each data hit in the local buffer pool accesses a "valid" copy of this particular data.  Specifying Cached Buffers as 0 and Average Cached Buffer Size as 0 will size for directory-only usage.

IMS/ESA OSAM Store-through cache structure can be used for caching data in addition to buffer invalidation. The structure size has to be large enough to hold directory entries for all buffers and data entries for all cached buffers.

Total Buffers:

Sum of the local buffers across all IMS systems sharing the coupling facility.  Include the peak number of OSAM buffers used at one time by IMS batch jobs.  Tally the subpool numbers from all IMS instances that were started using a DFSVSMxx member that specifies the structure of interest on its CFNAMES CFOSAM keyword.  It should include an estimate of the number of sequential buffering buffers in use.  By default, this is 40 buffers per online dependent region plus 40 for each batch job that might be running.

For example:

• IMS1: 3000 2K buffers, 4000 4K buffers, 40 sequential buffering buffers

• IMS2: 1000 2K buffers, 6000 4K buffers, 40 sequential buffering buffers

• IMS Batch cache (peak) 4000 4K buffers, 80 sequential buffering buffers (assuming a max of 2 batch jobs running simultaneously with 40 sequential buffering buffers each)

• Total as input = 3000 + 4000 + 40 + 1000 + 6000 + 40 + 4000 + 80 = 18160

Cached Buffers:

A subset or all of the OSAM buffers used by IMS Online subsystems and IMS Batch jobs may be stored in a Coupling Facility structure using the "IMS OSAM Data Caching" feature. Provide the peak number of buffers expected to be cached in the CF at one time.  This number will be used with the Average Cached Buffer size to determine the size of the cache portion of the structure.  Specifying a 0 for Cached Buffers along with specifying a 0 for Average Cached Buffer Size will size for directory-only usage.

Average Cached Buffer Size:

To find the average cached buffer size, calculate the total amount of storage used across all IMS Online and Batch (peak) systems for OSAM data caching buffers. Divide the total size by the total number of buffers, rounded up to the next 2K multiple.  Specifying a 0 for Average Cached Buffer Size along with specifying a 0 for Cached Buffers will size for directory-only usage.  Ignore sequential buffering buffers for purpose of this calculation.

For example:

• IMS1 OSAM cache: 1,000 2K buffers, 2000 4K buffers = 10,000K size

• IMS2 OSAM cache: 1,000 2K buffers, 2500 4K buffers = 12,000K size

• IMS Batch cache (peak): 2000 4K buffers = 8,000K size

• Total size (31,000K) divided by total number (8,500 buffers) = 3647

• Average cached buffer size equals 4096 (rounded up).